An ink jet printer produces images on an image receiving member by ejecting ink droplets onto the member as the member moves past a print head. The advantages of non-impact, low noise, low energy use, and low cost operation are largely responsible for the wide acceptance of ink jet printers in the marketplace.
Ink jet printers, however, may produce undesirable image defects in a printed image. One such image defect is non-uniform print density, such as “banding” and “streaking.” One major cause of “banding” and “streaking” is variation in the mass of the ink droplets ejected from different ink nozzles. These variations in ink mass may be caused by variations in the nozzles of a print head. The differences in the nozzles of a print head may be caused by deviations in the physical characteristics (e.g., the nozzle diameter, the channel width or length, etc.) or the electrical characteristics (e.g., thermal or mechanical activation power, etc.) of the nozzles. These variations are often introduced during print head manufacture and assembly.
The nozzles of a print head are typically arranged in arrays having rows and columns. Therefore, banding and/or streaking effects may occur in a horizontal or vertical line of an image. The variations in the ink drops that cause these defects relate to the density, size, or morphology of the ink drops that form an image. These variations can have a static (i.e., consistent) component and a random (i.e., non-consistent) component. Random variations between ink drops are generally less visible because their effects tend to cancel-out each other. The static variations are usually repeated more consistently and, thus, are more likely to be visible as banding or streaking defects.
Known techniques that adjust the driving signals of print heads to compensate for non-uniformity have been developed. Generally, these techniques involve a test pattern and/or patch being printed with one or more print heads and the test pattern and/or patch being imaged to obtain density measurements for the inkjet ejectors in the print heads. These density measurements are correlated to the individual inkjet ejectors in the print heads to enable a controller to adjust the firing signals used to actuate the inkjet ejectors. The controller modifies the firing signals for the inkjet ejectors to produce a more uniform density across the print heads. The technique is usually repeated until the density uniformity across a test pattern and/or patch does not vary from a target density by more than a predetermined threshold. Such adjustment techniques are typically known as print head normalization processes as the goal of the procedure is to normalize the inkjet ejectors to the target density.
The imaging of the test pattern and/or patch may be done in a variety of ways. In some printing systems, the media on which the test pattern and/or patch has been printed is removed from the printing system and placed in a scanner. The scanner typically moves an illumination source across the printed pattern and/or patch and optical sensors capture the response of the test pattern to the light. Darker areas cause the sensors to generate an electrical signal having a magnitude that is less than the magnitude of an electrical signal generated by a sensor receiving reflected light from a lighter area. Thus, the magnitude of the electrical signals may be used to identify the density or approximate drop mass of a droplet ejected by an inkjet ejector to form the test pattern and/or patch. The image generated by the scanner is processed by a computer to identify a drop mass for an inkjet ejector from the image pixel densities and to generate firing signal adjustments to normalize the inkjet ejectors. The firing signal adjustments may be stored in a memory and later downloaded into the printing system or transmitted over a network to the printing system.
In some printing systems, an illumination source and array of sensors are integrated within the printing system to image test patterns and/or patches generated by the printing system. These printing systems are able to generate test patterns and/or patches, capture images of the test patterns and/or patches with an inline sensor, and analyze the image data to measure the uniformity and determine firing signal adjustments to normalize the inkjet ejectors in a print head. These systems have the advantage of normalizing the inkjet ejectors without requiring an imaging and analysis system that is external to the printing system. Consequently, the control system for the printing system can obtain firing signal adjustments in situ.
In printing systems that are capable of imaging test patterns and/or patches within the system, the image environment affects the quality of the images. For example, the image receiving member has random structure that generates imaging noise in the imaging system. Likewise, surfaces that back or support the image receiving member may present structure that is transmitted through the image receiving member and captured in the test pattern image when the image receiving member is illuminated with light. Consequently, identifying image noise contributing structures and compensating the noise produced by the structure are worthwhile goals in printing systems that analyze test patterns and/or patches for inkjet ejector firing signal adjustments.